class Hash

  # lets through the keys in the argument
  # >> {:one => 1, :two => 2, :three => 3}.pass(:one)
  # => {:one=>1}
  def pass(*keys)
    tmp = self.clone
    tmp.delete_if {|k,v| ! keys.include?(k) }
    tmp
  end

  # blocks the keys in the arguments
  # >> {:one => 1, :two => 2, :three => 3}.block(:one)
  # => {:two=>2, :three=>3}
  def block(*keys)
    tmp = self.clone
    tmp.delete_if {|k,v| keys.include?(k) }
    tmp
  end

end